Multilayer optical films (i.e., films that provide desirable light transmission and/or reflection properties at least partially by an arrangement of microlayers of differing refractive index) are known and used in an ever increasing variety of applications.
Multilayer optical films have been demonstrated by coextrusion of alternating polymer layers. For example, U.S. Pat. No. 3,610,724 (Rogers), U.S. Pat. No. 4,446,305 (Rogers et al.), U.S. Pat. No. 4,540,623 (Im et al.), U.S. Pat. No. 5,448,404 (Schrenk et al.), and U.S. Pat. No. 5,882,774 (Jonza et al.) each disclose multilayer optical films. In these polymeric multilayer optical films, polymer materials are used predominantly or exclusively in the makeup of the individual layers. Such films are compatible with high volume manufacturing processes, and can be made in large sheet and roll formats. An illustrative embodiment is shown in FIG. 1.
In typical constructions, the film bodies comprise one or more layers of such multilayer optical films, sometimes referred to as an “optical stack”, and further protective layers on one or both sides thereof. Illustrative protective layers include, for example, so-called “skin layers” on one or both sides comprising more robust materials (e.g., polycarbonate or polycarbonate blends) which impart desired additional mechanical, optical, or chemical properties to the construction. U.S. Pat. No. 6,368,699 (Gilbert et al.) and U.S. Pat. No. 6,737,154 (Jonza et al.) disclose illustrative examples thereof. It is also common to include additional outer layers for protection (e.g., removable buffer layers sometimes referred to as “premask layers”) which protect the film body during early handling and processing and are then removed during later manufacturing steps. Illustrative examples include polyethylene-based films and polyurethane-based films. An illustrative embodiment is shown in FIG. 2.
Many product applications, however, require relatively small and sometimes numerous pieces of optical film. For these applications, small pieces of multilayer optical film can be obtained from a larger sheet of such film by subdividing the sheet by mechanical means, such as by cutting the sheet with a shearing device (e.g., scissors), slitting the sheet with a blade, or cutting with other mechanical apparatus (e.g., die stamping and guillotines). However, the forces exerted on the film by the cutting mechanism can cause layer delamination in a region along the cut line or edge of the film. This is particularly true for many multilayer optical films. The resultant delamination region is often discernable by a discoloration or other optical degradation relative to intact areas of the film. Because the multilayer optical film relies on intimate contact of the individual layers to produce the desired reflection/transmission characteristics, as a result of degradation in the delamination region it fails to provide those desired characteristics. In some product applications, the delamination may not be problematic or even noticeable. In other applications, particularly where it is important for substantially the entire piece of film from edge-to-edge to exhibit the desired reflection or transmission characteristics, or where the film may be subjected to mechanical stresses and/or wide temperature variations that could cause the delamination to propagate in the film over time, the delamination can be highly detrimental.
U.S. Pat. No. 6,991,695 (Tait et al.) discloses a method for using laser radiation to cut or subdivide optical films using, inter alia, removable liners to support the film and cut pieces. Though laser converting of polymeric materials has been known for some time, see, for example, U.S. Pat. No. 5,010,231 (Huizinga) and U.S. Pat. No. 6,833,528 (De Steur et al.), laser conversion of optical film bodies has not provided desired results. In the region of the optical body near the cutting zone (i.e., the edge) heat generated during the laser converting process often results in degradation of one or more components of the optical film body that impairs desired optical performance. The heat is often observed to disrupt the desired crystalline character of some layers in the optical film, making the component layers in such regions relatively amorphous in character such that desired birefringence is not achieved. As a result, the apparent color of the body in that region is not uniform to other portions of the body located more distantly from the cut zone. Further, the polycarbonate materials commonly used as skin layers tend to yellow upon exposure to the heat encountered during laser conversion, further impairing desired optical performance of the film.
There exists, therefore, a need for an improved method for subdividing multilayer optical film bodies and articles comprising such film. Preferably, the method would not produce delamination or color shifting or yellowing at the cut lines or film edges, would cut the film cleanly without substantial debris accumulation on the film, and would be compatible with automated and/or continuous manufacturing processes.